Sizing the drive system - Propulsion Systems for Hybrid Vehicles - page 157

fraction (EF) defined as the ratio of M/G peak power to total peak power. For
virtually all hybrid propulsion systems this fraction ranges from 0.1 < EF < 0.4.
At EF > 0.4, the vehicle electrical storage capacity must be increased to accom-
modate electric-only range, otherwise, the vehicle will not perform well on grades
or into strong headwinds without electric torque to augment the ICE.
Currently, many reasons are given for why hybridizing the battery is not
considered in many systems. Reasons cited include comments that combinations of
ultra-capacitors with batteries will be too expensive or the hybrid ESS will be too
large or weigh too much or be just too complex to manage and that the need for a
power electronic converter is simply out of the question. Just a sampling of recent
publications on the topic of ultra-capacitor plus battery combinations and of ultra-
capacitor-only mild hybrid vehicle performance [5] can be found in the reference
section of this paper. In every single instance the conclusion is the same, combi-
nation technologies outperform either type of energy storage component acting
alone. In this chapter the focus is on how model and simulation are applied early at
the design-in stage to give a specific case study of ultra-capacitors applied to the
battery storage system of an experimental battery electric vehicle (BEV). The
vehicle under consideration is a Mazda Miata that has been converted to electric
traction drive by Elite Power Solutions of Phoenix, AZ.
For the BEV case study evaluated here, the vehicle characteristics and added
battery mass are summarized in Table 4.5. The design in procedure followed
comprises three steps:

Step 1 . Simulate the full vehicle over one or more standard drive cycles. From
this simulation run(s) archive the resulting time step and vehicle propulsion
power, P ( V ), in a file. These data validate the peak power for acceleration and
that available from regeneration to size the ESS pack as well as the electric
traction drive inverter and electric machine.

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Step 2 . Import this P ( V ) file into a high performance dynamic simulation of the
battery and ultra-capacitor combination. The hybridized battery simulation can
be for both the tandem connection and more importantly for the actively par-
alleled combination. At this point any specific performance requests can be
incorporated such as including sufficient power boosting to support 50 kW for
5 s from the ultra-capacitor pack when at partial state of charge (SOC).

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Table 4.5 Vehicle parameters used in the performance simulations

Parameter Description Rating Parameter Description Rating

Battery pack 44S 1P 144 V, 28

kWh

Aerodynamic C d (#) 0.25

200 Ah

Battery mass kg 303.6 Rolling

resistance

C rr (kg/kg) 0.08

Vehicle mass kg 920 (1,127) Tyre rolling

radius

P205/50R16V
( r w ,m)

0.4

ICE power kW 125 Frontal area A f (m) 1.96